Effects of Nano Precipitates in Austenite on Ferrite Transformation Start Temperature during Continuous Cooling in Nb–Ti Micro-alloyed Steels

Abstract

In the present paper, transformation behavior during continuous cooling in non-deformed and hot deformed Nb–Ti micro-alloyed steels was investigated by using the thermal dilation method. Ti content in Nb–Ti bearing steels varied from 0 to 0.031 mass% with Nb content being kept to be constant. Thermal dilation curves were measured at different cooling rates, from which continuous cooling transformation (CCT) curves were built up. For non-deformed Nb–Ti steels, it was observed that ferrite transformation start temperatures (A_r3) decreased with increasing Ti content up to 0.015 mass%, leveled off in the range of 0.015 to 0.027 mass% Ti, and drastically decreased thereafter. For hot deformed Nb–Ti steels, A_r3 temperature did not exhibit significant difference with Ti addition lower than 0.027 mass%, and decreased drastically by further increasing Ti content. Austenite grain size (D_γ), Nb–Ti precipitates and residual strain were taken into account to explain the variation of A_r3 temperatures. Based on the experimental results, mathematical models for the calculation of A_r3 for non-deformed and hot deformed Nb–Ti micro-alloyed steels were developed.